I'm just a beginner and I spent an hour in the water (literally in the water, not on the water
) about a month ago in a 5/3 and I was fine. As the temperatures are going up now anything 4 should be ok when you get here.
As for IJburg. It's a somewhat limited spot that is, from what I've heard, only worth going with northerly winds.
That said, here is an overview of other spots http://www.kitehigh.nl/spots
When you click on them you get info about the wind directions that are right for the spot.